Nucleic acid hybridization studies using cloned retroviral DNA are being used to examine the sites of integration and organization of viral sequences in mammalian cells. One of the goals of these studies is to characterize the endogenous RNA tumor viral genomes in normal and malignant human tissues. A wide variety of these tissues have been examined for nucleic acid sequences related to various subclones of the endogenous baboon type C virus; the DNA of all normal and malignant tissues examined contain the same class of related sequences as detected by Southern blot analysis. The 12 kb and 5.5 kb fragments of normal human DNA that hybridize to the cloned baboon type C viral probe are being cloned in lambda vectors. Various Macaca mulatta (rhesus monkey) type C isolates have been compared by restriction enzyme analysis to the previously isolated type C virus from Macaca arctoides (stumptail macaque, MAC-1). Preliminary results indicate that the rhesus isolates can be distinguished from MAC-1. The organization of the two endogenous feline viruses (RD and FeLV) in various backcrossed animals which are offspring of a genetic cross between an F1 hybrid and a virogene-negative parent (leopard cat) is also being examined.